Nonlinear absorption in silicon nanocrystals

The nonlinear absorption of light in silicon nanocrystals suspended in glycerol is studied by the Z-scan method. The experimental data are used for calculating the nonlinear absorption coefficient β_Si-gl for silicon nanocrystals in glycerol (with a volume filling factor f = 2×10^-4), and the coefficient β_Si for pure silicon with a hypothetical volume filling factor f ≈ 1. For the laser radiation wavelength λ = 497 nm and the pulse duration τ = 0.5 ns, these coefficients are β_Si-gl = 1.2×10^-8 cm W^-1 and β_Si = 7.36×10^-5 cm W^-1, while the corresponding values for λ = 532 nm and τ = 10 ns are β_Si-gl = 5.36×10^-5 cm W^-1 and β_Si = 0.25 cm W^-1. Experiments with 540-nm, 20-ps laser pulses performed for two different filling factors equal to 2×10^-4 and 3×10^-3 gave nonlinear absorption coefficients β_Si-gl = 2×10^-7 and 3.6×10^-6 cm W^-1, respectively. Optical absorption and Raman scattering spectra of silicon nanocrystals are also studied. A theoretical analysis of the experimental results shows that optical absorption can be related to the localisation of photoexcited carriers in the conduction band. The localisation is caused by the action of strong static electric fields on an electron in a nanoparticle.